Broadly speaking, an electrophotographic image forming apparatus forms an image on a printing medium by performing sequentially the formation of an electrostatic latent image by scanning a light beam from a light scanning unit onto a photosensitive drum, the generation of a developed image by developing the electrostatic latent image using a developing agent such as a toner, the transfer of the developed image onto the printing medium, and the fixing of the transferred developed image on the printing medium.
A light scanning unit used in such an electrophotographic image forming apparatus deflection-scans a light beam by using a light deflector, e.g. a rotating polygonal mirror. In order for the deflected light beam to form an image on a surface being scanned, an imaging optical system is disposed between the light deflector and the photosensitive drum, A scan lens used in such imaging optical system is formed of either glass or plastic, and conventionally, in order to prevent the partial reflection of the light beam incident on the scan lens, has an anti-reflection coating on the incident surface of the scan lens. However, the anti-reflection coating may increase manufacturing costs and/or may adversely impact the light efficiency.